PLEISTOCENE TO HOLOCENE CLIMATE-MODULATED CHANGES IN FLUVIAL GEOMORPHOLOGY AND DETRITAL ZIRCON PROVENANCE IN CENTRAL TEXAS RIVERS

While detrital zircon (DZ) U-Pb provenance data are widely used to reconstruct hinterland tectonics and continental-scale drainage evolution, this study systematically couples changes in Quaternary fluvial geomorphology and DZ U-Pb provenance data of central Texas river systems to elucidate changes in climate, fluvial discharge, and locus of erosion in the absence of tectonics. We used geomorphologic and provenance analysis of Quaternary terrace and modern deposits of the Trinity and Colorado river systems to link variations in fluvial discharge and sediment provenance changes to climate-driven source-to-sink changes since the last glacial maximum. We mapped flights of terraces and modern river deposits using high-resolution digital elevation models created from LIDAR data. Fluvial channel geometries (e.g., bankfull depth and channel width) allowed for estimation of river discharge and changes through time. Initial measurements indicate much larger channel belts and discharge in the late Pleistocene. A decrease in discharge is accompanied by changes in relative sedimentary sources related to changes in erosional budgets and loci within the catchment area following the end of glaciation as deduced from DZ U-Pb data. The DZ data show that each river displays a time-varying diagnostic signature reflecting changes in contributing sedimentary sources through time. The Colorado river exhibits DZ age signatures dominated by a Grenvillian age mode, increasing from the late Pleistocene to the Holocene, while the Yapavai-Mazatzal (Y-M) age mode decreases abruptly, whereas the Trinity river is dominated by a Y-M and coupled Cenozoic age modes, which increase with time. For the Colorado, the abrupt decrease in Y-M, concomitant with a dramatic decrease in fluvial discharge, likely signals the end of glacial erosion in the Southern Rocky mountains, a shift to erosion of Precambrian lithologies, and recycling of Cenozoic lithologies in the lower drainage basin. For the Trinity, the dominant and increasing Y-M age mode indicates enhanced contribution from recycled Cenozoic lithologies. These DZ data results suggest a shift toward erosion and recycling of sedimentary sources in the lower portions of the drainage basins after the last glacial maximum.

These findings provide insights into fluvial drainage evolution in response climate change since the last glacial maximum of the latest Pleistocene. We expect similar fluvial responses to major fluvial network adjustments related to climate in analogous sediment-routing systems.